Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more co-axial cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit.
Such hammers are provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive for driving a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Rotary hammers can be employed in combination impact and drilling mode, and also in some cases in a drilling only mode, in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. A rotary drive mechanism transmits rotary drive from the electric motor to the spindle to cause the spindle, or a forwardmost part thereof to rotate.
Rotary hammers are known to have overload clutches in the drive train which transmits rotary drive from the motor to the spindle, or forwardmost part of the spindle. Such overload clutches are designed to transmit rotary drive when the transmitted drive torque is below a predetermined threshold and to slip when the transmitted drive torque exceeds the threshold. During rotary hammering or drilling, when working on materials of non-uniform hardness, for example aggregate or steel reinforced concrete, the bit can become stuck, which causes the torque transmitted via the rotary drive train to increase and causes the hammer housing to tend to rotate against the grip of the user. The torque can increase rapidly and in some cases the user can lose control of the hammer. The use of an overload clutch, can reduce the risk of this occurring, by ensuring that the clutch slips and rotary drive to the bit is interrupted at a torque threshold below that where a user is likely to lose control of the hammer. Accordingly, the clutch must slip reliably at a predetermined torque throughout the lifetime of the hammer, even after sustained use of the hammer.
It is known in some designs of hammer to locate the overload clutch around the spindle of the hammer as part of a spindle drive gear assembly. This generates a relatively compact design of overload clutch. The compactness of a rotary hammer is a critical design feature, in particular for smaller sizes of rotary hammer. The spindle drive gear is rotatingly driven by the motor pinion or by an intermediate shaft driven by the motor pinion and rotary drive is transmitted from the spindle drive gear to the spindle, or a forwardmost part of a spindle via the overload clutch.
In such a known design of overload clutch, the spindle drive gear is rotatably mounted on the spindle and a set of teeth on a side face of the spindle drive gear are engageable with a set of teeth on a facing side face of a clutch ring. The clutch ring is non-rotatably but axially slideably mounted on the spindle and is biased axially along the spindle into engagement with the spindle drive gear by a spring so that the sets of teeth engage. The spring is generally a strong helical spring which extends around the spindle over an axial distance between the clutch ring at one end of the spring and an end stop at the opposite end of the spring against which the spring bears. Below a predetermined threshold, the teeth are biased into engagement by the spring and torque is transmitted from the spindle drive gear to the spindle via the clutch ring. Above the predetermined torque the clutch ring can move against the force of the spring, and the sets of teeth ride over each other, and so the torque from the spindle drive gear is not transmitted to the spindle. Due to the axial movement of the clutch ring and the axially extending spring and the requirement for an end stop for the spring, this known overload clutch arrangement is not very compact and extends over a relatively long axial portion of the spindle. This problem with compactness is exacerbated where a spindle drive gear assembly incorporating such an overload clutch is arranged as a sub-assembly which sub-assembly can be moved axially along the spindle in order to move the spindle drive gear between different mode positions. In one mode position, for drilling only and/or rotary hammering, the spindle drive gear will mesh with the shaft or pinion which drives it and the spindle is rotated. In a second mode position, for hammering only, the spindle drive gear is moved axially along the spindle and out of engagement with the shaft or pinion and drive to the spindle is stopped.